Chapter 21: Integumentary System

Skin The skin is the largest organ in the body and has a dual origin: (1) A superficial layer, the epidermis, develops from the surface ectoderm. (2) A deep layer, the dermis, develops from the underlying mesenchyme. Epidermis Initially, the embryo is covered by a single layer of ectodermal cells (Fig. 21.1A). In the beginning […]

Chapter 20: Eye

Optic Cup and Lens Vesicle The developing eye appears in the 22-day embryo as a pair of shallow grooves on the sides of the forebrain (Fig. 20.1). With closure of the neural tube, these grooves form outpocketings of the forebrain, the optic vesicles. These vesicles subsequently come in contact with the surface ectoderm and induce […]

Chapter 19: Ear

Introduction In the adult, the ear forms one anatomic unit serving both hearing and equilibrium. In the embryo, however, it develops from three distinctly different parts: (1) the external ear, the sound-collecting organ; (2) the middle ear, a sound conductor from the external to the internal ear; and (3) the internal ear, which converts sound […]

Chapter 18: Central Nervous System

Introduction The central nervous system (CNS) appears at the beginning of the third week as a slipper-shaped plate of thickened ectoderm, the neural plate, in the mid-dorsal region in front of the primitive node. Its lateral edges soon elevate to form the neural folds (Fig. 18.1). Figure 18.1. A. Dorsal view of a late presomite […]

Chapter 17: Head and Neck

Introduction Mesenchyme for formation of the head region is derived from paraxial and lateral plate mesoderm, neural crest, and thickened regions of ectoderm known as ectodermal placodes. Paraxial mesoderm (somites and somitomeres) forms a large portion of the membranous and cartilaginous components of the neurocranium (skull) (Fig. 17.1; see also Chapter 10 and Fig. 10.6), […]

Chapter 16: Urogenital System

Introduction Functionally, the urogenital system can be divided into two entirely different components: the urinary system and the genital system. Embryologically and anatomically, however, they are intimately interwoven. Both develop from a common mesodermal ridge (intermediate mesoderm) along the posterior wall of the abdominal cavity, and initially, the excretory ducts of both systems enter a […]

Chapter 15: Digestive System

Divisions of the Gut Tube As a result of cephalocaudal and lateral folding of the embryo, a portion of the endoderm-lined yolk sac cavity is incorporated into the embryo to form the primitive gut. Two other portions of the endoderm-lined cavity, the yolk sac and the allantois, remain outside the embryo (Fig. 15.1A,B,C–D). Figure 15.1.Sagittal […]

Chapter 14: Respiratory System

Formation of the Lung Buds When the embryo is approximately 4 weeks old, the respiratory diverticulum (lung bud) appears as an outgrowth from the ventral wall of the foregut (Fig. 14.1A). The appearance and location of the lung bud are dependent upon an increase in retinoic acid (RA) produced by adjacent mesoderm. This increase in […]

Chapter 13: Cardiovascular System

Establishment and Patterning of the Primary Heart Field The vascular system appears in the middle of the third week, when the embryo is no longer able to satisfy its nutritional requirements by diffusion alone. Progenitor heart cells lie in the epiblast, immediately adjacent to the cranial end of the primitive streak. From there, they migrate […]

Chapter 12: Limbs

Limb Growth and Development The limbs, including the shoulder and pelvic girdles, comprise the appendicular skeleton. At the end of the fourth week of development, limb buds become visible as outpocketings from the ventrolateral body wall (Fig. 12.1A). The forelimb appears first followed by the hindlimb 1 to 2 days later. Initially, the limb buds […]